1. Field of the Invention
This invention relates to tires having an improved wear resistance, and more particularly to a pneumatic tire which considerably improves a wear resistance at a higher severity region by compounding with polybutadiene having a high content of trans bond unit.
2. Related Art Statement
In tires for truck and bus, the wear resistance is considerably improved in accordance with the recent radial formation and also failures such as heat separation and the like are rapidly reduced, so that these tires are used under a high severity region. The term "severity" used herein means a degree of tire service condition, which is dependent upon various factors such as load, speed, temperature, road surface roughness, running state, acceleration distribution in front, back and lateral directions at running state and the like and is subjected to complicated interaction of these factors. As an indication of total severity viewed from rubber properties, there is frequently used a running distance (KPM) per unit groove depth in tread. Since the high severity is not more than 10,000 km/mm in the conventional KPM, as a planning means for rubber compounding applicable to pneumatic tires used under such a condition, there are generally conducted a method of using a blend of natural rubber and cispolybutadiene rubber or styrene-butadiene rubber as a tread rubber, a method of using an increased amount of carbon black having a fine grain size and the like.
As described in Rubber Chemistry and Technology, vol. 44, page 996 (1971), it is known that the wear resistance is considerably improved at the high severity region by using high-cis polybutadiene alone. In this case, the wear resistance in the high-cis polybutadiene is really 2 times or more as compared with that of SBR 1712 as a control. However, when using the high-cis polybutadiene alone or in a great amount, the processability is considerably degraded but also the fracture properties are poor, so that the chipping, rib tear and the like are caused and the braking performance on wet road surface is undesirably damaged. Therefore, it is usually attempted to use an isoprene series high strength polymer in an amount of not less than 25 parts by weight to polybutadiene rubber as a rubber blend as proposed in Japanese Patent Application No. 63-68434.